This invention relates to a method and system for real-time sensing of hole depth when a laser beam is drilling the surface of a workpiece. More specifically, this invention relates to depth sensing by sensing the magnetic field from ionized plasma caused by the laser beam hitting the workpiece.
Lasers are sometimes used for drilling holes in various parts. When using a high-power laser beam to drill a hole in a workpiece, it is useful to know the depth of the hole. Further, the real-time control of hole depth would be advantageous.
Various arrangements have been used for the different types of sensing of holes in laser drilling. Among those are arrangements which detect when a laser has broken through a wall of a workpiece. Phototransistor-type detectors rely upon having the detectors oriented towards the backside (i.e., side opposite the drill side upon which the laser beam is directed) of the workpiece. When the breakthrough occurs, the detectors sense the light and provide a signal indicating that breakthrough has occurred.
Among prior patents related to the laser drilling or laser machining of workpieces such as machine parts in U.S. Pat. No. 4,608,480, issued Aug. 26, 1986, to Bizot et al. That patent discloses control of the operating parameters of a laser by sensing the sounds emitted by the melting of the workpiece.
U.S. Pat. No. 4,504,727, issued Mar. 12, 1985, to Melcher et al. discloses a laser drilling system having feedback control based upon sensing an acoustic signal arising from subjecting the workpiece to a laser.
The Kasner et al. U.S. Pat. No. 4,789,770, issued Dec. 6, 1988, shows an arrangement for controlling depth in a laser drilling system. An optical sensing arrangement is used to detect the depth of the hole which is drilled.
The Inoue U.S. Pat. No. 4,689,467, issued Aug. 25, 1987, shows a laser machining apparatus where a source of plasma is used in combination with the laser. Additionally, infrared rays radiated from the workpiece may be sensed in order to automatically adjust the laser beam.
The Ortiz, Jr. et al. U.S. Pat. No. 4,764,655, issued Aug. 16, 1988, shows a laser materials processing system including an arrangement to determine the beam diameter by imaging techniques.
The Anthony U.S. Pat. No. 4,473,737, issued Sept. 5, 1984, shows a laser drilling technique whereby holes are drilled in a transparent or substantially transparent material (i.e., transparent to the laser radiation) with the hole starting on the side of the workpiece or machine part opposite the side on which the laser beam impinges.
R. Koegl and R. Hogle, co-inventors of the present application, were inventors of U.S. application Ser. No. 07/556,678, filed July 23, 1990 and entitled "Real-Time Magnetic-Flux Breakthrough Detection Method and System for Laser Drilling", assigned to the assignee of the present application, and hereby incorporated by reference. That application disclosed an inductive sensing arrangement for detecting when a laser beam has drilled a hole completely through a wall of a workpiece.
Although various known techniques for controlling laser machining processes have been useful, they have often been subject to one or more of several disadvantages, such as inaccuracy, high cost and complexity. Those techniques which provide for breakthrough detection are useful in some applications, but are of little value in applications where one wants to control the depth of a hole which does not go completely through a wall of a workpiece or machine part. Those techniques which use optical sensing of the drilling operation from the front or drill side of a workpiece may fail to operate properly because of the various debris which is sprayed outwardly on the drill or front side of the workpiece.